1. Field of the Invention
The present invention relates generally to the field of induction resistivity measurements using downhole induction tools for the evaluation of potential hydrocarbon bearing formations. The present invention relates to a method and apparatus that utilizes compact magnetic sensors for multi-component induction and micro-resistivity measurements with sufficient immunity to adverse effects of such parasitic eddy currents flowing on the surface of a down hole induction tool.
2. Summary of the Related Art
Electromagnetic induction resistivity tools are well known in the art. These induction resistivity well logging tools are used to determine the electrical conductivity, or its converse, the electrical resistivity, of earth formations, which are penetrated by a borehole. In the past, formation conductivity has been determined based on the results of measuring a magnetic field caused by eddy currents induced in the formation by a well logging tool. The resulting electrical conductivity measurements are used for, among other purposes, inferring the fluid content of earth formations. Typically, lower conductivity measurements (or higher resistivity) indicate a hydrocarbon-bearing layer in a formation. The physical principles of electromagnetic induction well logging are described in J. H. Moran and K. S. Kunz, Basic Theory of Induction Logging and Application to Study of Two-Coil Sondes, Geophysics, vol. 27, no. 6, part 1, pp. 829-858, SOCIETY OF EXPLORATION GEOPHYSICISTS, December 1962. Numerous improvements and modifications have been devised to the basic electromagnetic induction resistivity tools as described in the Moran and Kunz reference, supra. Some of these improvements are described, in U.S. Pat. No. 4,837,517 issued to Barber, U.S. Pat. No. 5,157,605 issued to Chandler et al. and in U.S. Pat. No. 5,600,246 issued to Fanini et al.
The conventional induction well logging tool is lowered into a borehole drilled into a hydrocarbon bearing formation in the earth. The typical resistivity well logging tool further consists of a sensor section and other equipment for acquiring resistivity data regarding the formation. A determination is made from the data regarding the physical parameters that characterize the formation surrounding the well bore. The typical sensor section, or mandrel, comprises one or more induction transmitters and receivers positioned along the tool's longitudinal axis. Electrical circuitry is provided to generate and apply an electrical voltage to the transmitter's induction coil. Circuitry is also provided to condition and process signals emanating from the formation, that are picked up by the receiver induction coils. The acquired data is then stored in the tool or sent to the earth's surface by means of telemetry or a wire line, which is also used to lower the tool into the borehole.
In general, the individual layers in a multi-layered or laminated hydrocarbon-bearing zone are difficult to detect when using a conventional induction well logging resistivity tool with transmitter and receiver induction coils having magnetic moments oriented only along the borehole axis. These multi-layered zones usually consist of thin alternating sand and shale layers. Oftentimes, the layers are so thin that a conventional logging tool with limited resolution cannot individually detect a layer. In this case, only the average conductivity of the multi-layer zone can be evaluated.
To address this problem, well loggers began using transverse induction logging tools to measure the conductivity of multi-layer zones. Transverse tools provide transmitters and receivers positioned so that their magnetic moments are oriented transverse to the tool's longitudinal axis. One such transverse induction well logging tool has been described in U.S. Pat. No. 5,781,436 issued to Forgang et al.
The response of transverse coil arrays is also determined by an average conductivity, however, it is the relatively lower conductivity of hydrocarbon-bearing sand layers that dominate this transverse coil conductivity estimation. The volume of shale/sand in multi-layer zone of a formation can be determined from measurements taken with a gamma ray or nuclear well logging tool. Consequently, the conductivity of individual shale and sand layers can be determined from a combination of the conventional and transverse induction logging tool measurements.
One of the main difficulties in interpreting the data acquired by a transverse induction logging tool is its vulnerability to the presence of a conductive well fluids in the bore hole. This condition is known as the well bore fluid invasion effect. A known method for reducing the unwanted impact of well bore fluid invasion effects on a transversal induction logging tool response was disclosed in L. A. Tabarovsky and M. I. Epov, Geometric and Frequency Focusing in Exploration of Anisotropy Seams, Nauka, USSR ACADEMY OF SCIENCE, Siberian Division, Novosibirsk, pp. 67-129 (1972) and also in L. A. Tabarovsky and M. I. Epov, Radial Characteristics Of Induction Focusing Probes With Transverse Detectors In An Anisotropic Medium, SOVIET GEOLOGY AND GEOPHYSICS, 20 (1979), pp. 81-90.
The Tabarovsky method uses a transverse induction logging tool. In the Tabarovsky method, the transverse induction transmitter induces currents into the formation adjoining the borehole by irradiating a magnetic field into the formation. In turn, induction tool receivers measure the magnetic field response associated with the currents induced in the formation. To enable a wide range of vertical resolutions and to facilitate effective suppression of the unwanted borehole effects, measurements of the magnetic field from the formation can be obtained at different separations or distances between the transmitter and receiver. However, even with these measurement techniques, the data logs obtained with a conventional transversal induction logging tools can be inconsistent and difficult to interpret. Moreover, these problems are exacerbated when logging through a sequence of varying conductivity layers.
In induction logging, the quality of acquired data primarily depends on the electromagnetic environment in which the logging tool operates. Thus, in an ideal case, the logging tool measures only those signals associated with the eddy currents induced in the formation by the primary magnetic field emanated by the induction tool transmitter. Variations in the magnitude and phase of the induced eddy currents occur in response to variations in formation conductivity. These variations are reflected as variations in the output voltage of induction receivers located in the induction tool. In conventional induction tools, these receiver voltages are conditioned and processed using analog phase sensitive detectors or processed algorithmically. The processing allows for determining both receiver voltage amplitude and phase with respect to the induction transmitter current or its magnetic field waveform.
There are also known Measurement While Drilling (or MWD) induction tools which typically utilize solenoid-type transmitter and receiver induction coils coaxial with the tool mandrel. These tools produce a “classical” set of induction measurements in the propagation mode. The tools measure attenuation and phase shift in the transmitted magnetic field due to the influence of the adjacent formation characteristics. Known MWD induction tools utilize two coaxial receiver coils positioned in the center of the mandrel and two sets of balanced transmitter coils on both sides of the receiver coils. This balanced coil configuration, when operating at two frequencies of, for example, 400 kHz and 2 MHz, enables the typical known induction logging tool to quantitatively evaluate eight different directional horizontal formation resistivities for a plurality of investigation depths. These known MWD induction tools, however, utilize sensors that are relatively large so that pad mounting of the sensor is not possible. Conventional sensor have difficulty operating in both conductive and nonconductive oil-based mud. Thus there is a need for a compact induction logging tool sensor that can be pad mounted and operates in both conductive (water-based) and nonconductive (oil-based) mud.
An induction tool's total magnetic moment is defined by the effective geometric area of the transmitter and receiver coils and by the transmitter current. The effective area of a particular coil has been determined by coil dimensions themselves and presence of conductive tool parts in proximity to the coil. By design, the transmitter and receiver coils are built with an effective geometrical area sufficiently large to achieve the maximum possible random noise-free measurements. A coil's geometric area is sufficiently large when non-productive energy losses can be adequately compensated. Such losses may occur in the tool while generating the primary magnetic field or detecting the secondary magnetic field flux induced from the formation. In many circumstances these limitations can be evaluated from respective tool numerical modeling and laboratory tests and then be corrected or calibrated.
It is well known in electromagnetic field theory that an externally generated alternating magnetic field radiated normal to the surface of a conductive body will induce eddy currents on this surface. These eddy currents, in turn, produce own magnetic fields that, according to Faraday's Law of induction, opposes the external field which induced them. Generally, the opposing field magnitude increases with increasing surface electrical conductivity and decreases with increasing distance from the source of the radiated magnetic field. For those who are skilled in the art it should be understood that an integral magnetic flux when in close proximity to a highly conductive metallic surface becomes negligible. Similar effects are present in induction tools having transmitter and receiver coils wound in a close proximity to the metal tool body.
Hence, when a transmitter induction coil is positioned closely above a conductive tool surface, the magnetic field of eddy currents induced on the tool surface oppose the primary source and thus decreases the integral flux radiated into the formation. Conversely, if a receiver induction coil is positioned proximately above a conductive tool surface, the magnetic field of eddy currents induced on this surface due to formation response will decrease the effective flux crossing the receiver coil. Both processes substantially reduce receiver output voltage and, consequentially, lower the tool's signal-to-noise ratio.
In the most practical tool designs, the transmitter field wavelength is significantly longer than the electromagnetic skin depth of the tool body material and the transmitter coil's linear dimension. This fact allows considering presence of a 180 degree phase shift between the transmitter and surface currents magnetic fields, therefore, neglect reactive electromagnetic losses in the tool body materials. In this situation presence of conductive tool parts in proximity to the sensor's coils results primarily in reducing an overall tool magnetic moment. However, with a further increase of operating frequencies the transmitter magnetic field wavelength becomes comparable with tool assembly dimensions and tool body reactive losses appear. Having reactive losses in the instrument significantly complicates both the shape of transmitted field and frequency response of the receiver. Thus, the eddy currents effect can become complex and their effect should be analyzed separately for every logging case. Thus there is a need for a method and apparatus for shaping the transmitted field and for efficiently dealing with eddy currents induced in the tool.
Parasitic magnetic coupling between the formation and the conductive metal parts of the downhole tool creates problems associated with another kind of parasitic eddy current induced on the tool body surface. Generally a high degree of magnetic coupling is desired between the tool transmitter and the formation. High magnetic coupling is also desirable between the tool receiver and the formation. This high magnetic coupling facilitates increased tool efficiency and increases the overall tool signal to noise ratio. The presence of conductive bodies typically found in the mandrel of known induction tools, however, becomes problematic. The conductive bodies result in the appearance of additional undesirable magnetic coupling between these conductive bodies and different sources of electromagnetic radiation/reception inducing parasitic eddy currents.
These parasitic eddy currents flow on the surface of the downhole induction tool and produce undesirable magnetic fields that adversely interact with resistivity measurements as additive error components. These induced magnetic fields couple directly into the receiver coil(s) and create undesirable offsets in measurement signals. Moreover, eddy currents due to formation-to-tool body coupling are temperature and frequency dependent, making adverse effects due to such parasitic eddy currents difficult, if not impossible to account for by known methods. Thus, there is a need for a method and apparatus that utilizes compact magnetic sensors for multi-component induction and micro-resistivity measurements with sufficient immunity to adverse effects of such parasitic eddy currents flowing on the surface of a down hole induction tool.